The present invention relates to a multistage hydraulic machine and a method of controlling operation thereof as a pump or turbine so as to improve hydraulic performance under a steady operating condition and stably operate the hydraulic machine by suppressing vibration and noise generated therein.
Generally, the operation or running condition of a hydraulic machine can be controlled by adjusting the amount of water passing through the hydraulic machine, this adjustment being done by guide vanes provided on the periphery of a runner or an inlet valve located at an inlet portion of the hydraulic machine. Regarding a multistage hydraulic machine in which respective pressure stages are provided with runners connected through return passages respectively, there have been proposed several methods for controlling water flow conditions at the respective stages by guide vanes located on the peripheries of runners of the stages, thereby controlling the operation of the conventional multistage hydraulic machine. However, with the multistage hydraulic machine of the type described above, it is extremely difficult, because of constructional limitations, to provide guide vanes on the outer peripheries of the runners of the corresponding stages and to control the operating or closing of the guide vanes by a guide vane control unit which is connected to the guide vanes, which provides a problem for practical use of the multistage hydraulic machine. Thus, in the multistage hydraulic machines of this kind, in many cases only stationary guide vanes having a constant degree of opening are located on an outer peripheral surface of a runner of each stage, and adjustment of the amount of water flow and control of the operation of the machine are performed by open-close control of an inlet valve provided at an inlet portion of the hydraulic machine. Moreover, in such multistage hydraulic machines, the water flow condition at the periphery of the runner of each stage cannot be properly adjusted in response to the amount of water flow passing therethrough, so that in low and high flow-rate ranges apart from design points, hydraulic performance decreases and the hydraulic machine is itself operated under lowered total hydraulic performance. In addition, water head loss at a portion near the inlet valve is increased by adjusting the amount of water passing through the inlet valve by partially closing the opening of the valve, and therefore, hydraulic energy corresponding to the water head loss is wasted and the running efficiency of an electric power plant is decressed.
In order to obviate defects or problems described above, there has also been proposed a method for controlling the operation of a multistage hydraulic machine, in which movable guide vanes are provided only for the highest pressure stage and the amount of water flow can be adjusted by regulating the degree of opening of these movable guide vanes. According to this operation control method, the water head loss at the inlet valve which is observed in a case where no movable guide vanes are arranged at the highest pressure stage was not observed, so that this control method has a better operation efficiency than the first mentioned method. However, regardless of such advantage as described above, in the later mentioned method, since the lowest pressure stage is operated at an operation point largely apart from a designed operating point during operation as a turbine with a small amount of water, water separation phenomenon and/or secondary local flow may often occur, and these adverse phenomena are often observed as water pressure on the outlet side of a runner lowers, so that there are problems such as an increase in water separation phenomenon and secondary local flow at the lowest pressure stage where the water pressure on the outlet side of the runner of this stage is lower than those of the other stages, and the hydraulic machine of this type is operated under an unstable condition with cavitation, strong vibrations and noises.
Consequently, as described hereinabove, a multistage hydraulic machine having pressure stages each provided with movable guide vanes whose degrees of openings can be adjusted is most preferred for hydraulic characteristics, but imposes severe restriction on the construction of a machine for practical use. A multistage hydraulic machine, in which respective pressure stages are provided with stationary guide vanes whose degrees of openings cannot be adjusted and water flow control is done only by adjusting an inlet valve, has the most simple structure, but has extremely less hydraulic characteristics. Moreover, a multistage hydraulic machine in which only the highest pressure stage is provided with movable guide vanes has been proposed, but cavitation, vibrations and noises were often generated when the machine was operated with a small amount of water.